Cells utilize the dynamic addition and removal of SUMO, a small ubiquitin-like modifier to modulate protein function. SUMO protein modification exist in yeast, humans and possibly all eukaryotes. In humans, dysregulation of SUMO dynamics plays a role in certain forms of leukemia and prostate cancer. Furthermore, several gene products that are involved in cancer pathogenesis are regulated by the addition and removal of SUMO. Production of mature, conjugation competent SUMO as well as desumoylation substrates depends on proteases of the Ulp family. Mammalian cells have at least 7 Ulp-like molecules. The founding members of the Ulp family are the yeast Ulp1 and Ulp2 proteins, two SUMO proteases that were identified in the Hochstrasser lab. Absence of Ulp1p mediated desumoylation leads to mitotic arrest and cell death in budding yeast. Despite a growing number of known sumoylated proteins in yeast, little is known about which individual substrates Ulp1 must desumoylate in order to ensure proper cell cycle progression. Since Ulp1p localizes to the nuclear pore complex we hypothesize that this SUMO protease may help to control important cell cycle regulators in transit across the nuclear envelope. The main goal of this project is to identify evolutionarily conserved proteins that interact with Ulp1p and may be important for the cell division cycle. To achieve this goal cell biological, genetic, and biochemical approaches will be used. Specifically, 1) the exact time at which Ulp1p function is required during mitosis will be determined using cell biological techniques. 2) Using a ulp1 mutant ULP1 interactors will be isolated genetically and characterized. 3) The function of a novel Ulp1 interactor that can bind SUMO, Hex3, will be analyzed. In particular, our studies are designed to understand the functional roles of Ulp1p and to lay the framework for future studies of similar situations and homologous proteins in mammalian cancer models. PUBLIC HEALTH RELEVANCE: Budding yeast cells expressing a defective SUMO protease enzyme (Ulp1) exhibit aberrant mitosis and altered cell cycle progression. In humans these phenomena are linked to spontaneous abortions, ageing and cancer. Studying the ulp1 mutant has helped us to identify some of the responsible molecules.